What you have there is a 600 trap. A 600 field in that condition should be around $700.00. However, a trap model will be harder to sell because of the advancement of competition guns over the last 10 years --- Mobile chokes/recoil reducers/back boring/lengthened forcing cones and so on...... These guns are very well made and I've never seen one shoot loose --- very few issues -- built like a tank. Anyway after that ramble I would estimate it at $550 to 600 for an entry level trapper.
Cheers, John